Circuit probe for measuring a differential circuit

ABSTRACT

A circuit probe (300) is provided for measuring a differential circuit. The circuit probe (300) incorporates a novel transmission line (325) which is formed to propagate signals with substantially equal even-mode and odd-mode characteristic impedances. The circuit probe (300) further includes signal contacts (314, 316) and ground contacts (312, 318) coupled to signal conductors (334, 336) and ground conductors (332, 338), respectively, of the transmission line (325).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Ser. No. 08/270,356, Docket No.CM01685J, concurrently filed on Jun. 29, 1994, entitled "METHOD ANDAPPARATUS FOR CHARACTERIZING A DIFFERENTIAL CIRCUIT", by David E.Bockelman and William R. Eisenstadt and assigned to Motorola, Inc. nowU.S. Pat. No. 5,495,173.

TECHNICAL FIELD

This invention relates generally to differential circuits, and moreparticularly, to the characterization of differential circuits throughmeasurements.

BACKGROUND OF THE INVENTION

There has been an emerging need to isolate separate circuits constructedon the same integrated circuit. For example, it may be necessary toisolate a circuit with high sensitivity, such as an analog circuit, froman circuit having high transient noise, such as a digital circuit.Traditional single-ended circuits have been replaced with differentialcircuits in some low-frequency applications where such concerns havearisen. Differential circuits can provide better isolation in someapplications as certain types of interferences may be greatly reduced.However, differential circuits are more difficult to design and testthan typical single-ended circuits, particularly when such differentialcircuits are designed to process signals carried on radio and microwavefrequencies. Traditional methods of testing differential circuits havebeen based on measuring voltages and currents. For example, in a typicaltest environment for integrated circuits, test probes engage ports oncircuit wafers to determine voltages and currents between two points ona differential circuit. The use of voltages and currents fail at radiofrequencies and at microwave frequencies because of difficulties inproviding accurate measurements.

Scattering parameters have been developed for single-ended N-portcircuits. No corresponding characterization parameters have beendeveloped to fully characterize radio frequency and microwave frequencybased differential circuits. Currently, it is possible to performpartial measurements of differential modes using a balanced probe,manufactured by CASCADE MICROTECH. However, this technique does notallow for full characterization of the differential circuit. The priorart lacks both an adequate methodology and apparatus to properlycharacterize radio and microwave frequency differential circuits.Therefore, a new measurement technique for characterizing radio andmicrowave frequency differential circuits is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a two-port differential circuit.

FIG. 2 is a block diagram of a measurement system, in accordance withthe present invention.

FIG. 3 is a fragmentary top view of a circuit probe showing the tipportion, in accordance with the present invention.

FIG. 4 is a second fragmentary view of the circuit probe of FIG. 3showing the bottom or interface surface.

FIG. 5 is an enlarged fragmentary view showing dimensionalcharacteristics of a transmission line of the circuit probe of FIG. 3.

FIG. 6 is a fragmentary cross-sectional view of the circuit probe ofFIG. 3.

FIG. 7 is a graph of a design parameter for a transmission line based onthe ratio of the width of a transmission line signal conductor to theseparation between the conductor and an underlying ground plane, and therelative dielectric constant of the substrate separating the conductorand ground plane.

FIG. 8 is a graph of a design parameter for a transmission line based onthe ratio of the width of a transmission line signal conductor to thespacing between coupled signal conductors, and the dielectric constantof the substrate separating the conductor and ground plane.

FIG. 9 is a graph of a design parameter for a transmission line based onthe ratio of the width of a transmission line signal conductor to thespacing between the conductor and a coplanar ground plane, and therelative dielectric constant of the substrate separating the conductorand ground plane.

FIG. 10 is a representation of procedures used in characterizing adifferential circuit, in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally, the present provides a method and apparatus forcharacterizing differential circuits. Scattering parameters(S-parameters) have been developed for differential circuits, and ameasurement system defined for determining the S-parameters. In thepreferred embodiment, a switching network provides differential andcommon mode signals for input to a differential circuit under test. Theinput signals are propagated over a coupled transmission line pair withsubstantially equal even and odd mode characteristic impedances andthrough a circuit probe to the differential circuit. Measurements areconducted at one or more ports of the differential circuit to determinesignal propagation characteristics. Scattering parameters are thendetermined from the input signals, and the measured output signals tocharacterize the differential circuit.

DIFFERENTIAL CIRCUITS

Referring to FIG. 1, a simplified differential circuit 100 is shown, asmay be developed for a radio frequency (RF) or microwave frequencyapplication, in accordance with the present invention. Pairs of coupledtransmission lines 110, 120 provide input and output to the differentialcircuit 100. As is typical in a differential circuit, differential modesignals are described by a difference in voltage and current flowbetween the individual lines in a coupled pair, where the difference involtage (.increment.V₁, .increment.V₂) is not equal to zero. Using thesedefinitions, a signal propagating between the lines of the coupled pairs110, 120 is not referenced to a ground potential, but rather to eachother, and thus the term differential mode signal. This differentialmode signal propagates in a transverse electro-magnetic (TEM), orquasi-TEM fashion with a well-defined characteristic impedance andpropagation constant. The coupled line pairs 110, 120 allow propagatingdifferentials signals to exist. In the preferred embodiment, thedifferential circuit 100 has two ports 112, 116, however, the conceptsof the present invention is applicable to N-port circuits.

As in most practical implementations of differential circuits, a groundplane 130, or some other global reference conductor, is incorporatedeither intentionally or unintentionally. This ground plane 130 allowsanother mode of propagation to exist, namely common mode propagation.Generally, the common mode wave applies equal signals with respect toground at each of the individual lines in a coupled pair, such that thedifferential voltage is 0, i.e., .increment.V₁ =.increment.V₂ =0.According to the present invention, both common mode and differentialmode signals are treated as propagating simultaneously, and these modesare used to properly characterize the differential circuit. Thus, ingeneral, a signal traveling along a coupled transmission line pairwithin a differential circuit is a combination of differential modewaves and common mode waves. This combination is referred to asmixed-mode propagation. From this mixed-mode propagation, scatteringparameters (S-parameters) are defined which adequately characterizes thedifferential circuit.

MIXED MODE "S" PARAMETERS

Before defining the S-parameters, additional definitions are needed toestablish a reference base. The differential mode voltage (V_(dm)) at apoint, x, is defined to be the difference between voltages on node(1)102 and node(2) 104:

    V.sub.dm (x)=V.sub.1 -V.sub.2.

This standard definition establishes a signal that is no longerreferenced to ground. In a differential circuit, one would expect equalcurrent magnitudes to enter the positive input terminal as that leavingthe negative input terminal. Therefore, the differential mode current(I_(dm)) is defined as one-half the difference between currents enteringnode(1) 102 and node(2) 104.

    I.sub.dm (x)=1/2(I.sub.1 -I.sub.2).

The common mode voltage in a differential circuit is typically theaverage voltage of the port. Hence, common mode voltage (V_(cm)) is 1/2the sum of the voltages on node(1) 102 and node(2) 104:

    V.sub.cm (x)=1/2(V.sub.1 +V.sub.2).

The common mode current (I_(cm)) at a port is the total current flowinginto the port, i.e.,

    I.sub.cm (x)=I.sub.1 +I.sub.2.

The characteristic impedance of each mode (Z_(dm), Z_(cm)) can bedefined as a ratio of the voltage to current of the appropriate modes atany point, x, along the line:

    Z.sub.dm =V.sub.dm (x)/I.sub.dm (x);

    Z.sub.cm =V.sub.cm (x)/I.sub.cm (x).

As is generally known in the art, a symmetrical coupled pair oftransmission lines can support two fundamental modes--an even mode andan odd mode. Additionally, there are even and odd mode characteristicimpedances associated with the even and odd modes of signals propagatingon a symmetrical coupled transmission line pair with respect to ground.The mode characteristic impedance for the even mode is termed Z_(e), andfor the odd mode, Z_(o). The characteristic impedances of thedifferential mode and common mode can be related to that of the odd modeand even mode, respectively, by the following equations:

    Z.sub.dm =2Z.sub.o ;

    Z.sub.cm =Z.sub.e /2.

As earlier stated, the preferred embodiment provides for analysis of atwo-port differential circuit, and as such, normalized wave equationshave been developed. Imposing the condition of low-loss transmissionlines on the coupled pair, the characteristic impedances areapproximately purely real. Under this condition, Z_(dm) ≈R_(dm), andZ_(cm) ≈R_(cm), where R_(dm) and R_(dm) are the real portions of Z_(dm)and Z_(cm), respectively. Accordingly, the normalized wave equations atthe first port 112 can be stated as: ##EQU1##

An analogous analysis will yield the normalized equations for the secondport 116 using the corresponding voltages (V₃, V₄) and currents (I₃, I₄)at node(3) 106 and node(4) 108. Similarly, parameters can be developedfor an N-port differential circuit.

Generally, the mixed-mode S-parameters may be defined as:

    [b]=[S][a]

where [a] and [b] denote n-dimensional column vectors, and [S] denotesan n-by-n matrix. For a two-port application, the generalized mixed-modetwo-port S-parameters can be stated as: ##EQU2## where [S_(dd) ] is a2-by-2 matrix representing the differential S-parameters, [S_(cc) ] is a2-by-2 matrix representing the common mode S-parameters, [S_(dc) ] and[S_(cd) ], are the mode conversion or cross mode S-parameters. Inparticular, [S_(dc) ] describes the conversion of common mode waves intodifferential common mode waves, and [S_(cd) ] describes the conversionof differential mode waves into common waves.

Having presented some of the theoretical aspects of the presentinvention, the S-parameter measurement system of the preferredembodiment will now be described.

Referring to FIG. 2, a block diagram of a measurement system 200 isshown, in accordance with the present invention. The measurement system200 includes a two-port network analyzer (NWA) 208, a switching network212, and a pair of circuit probes 202, 204. The measurement system 200operates under the control of a controller 205. The switching network212 is coupled to ports 209, 210 on the NWA 208 via a two-positionswitches 270, 272. The switch 270 is coupled to a pair of switches 280,283. Switch 280 is coupled to a 180° signal splitter/combiner 222 and a0° splitter/combiner 224. In the 180° splitter/combiner 222, thesplitter function splits a signal into two signal components havingsubstantially equal amplitudes, and having a phase difference ofsubstantially 180 degrees. Conversely, the 180° combiner functionsubstantially substracts two periodic signals resulting in a singlesignal. In the 0° splitter/combiner 224, the splitter function splits asignal into two signal components having substantially equal amplitudesand having substantially equal phase. The 0° combiner functionsubstantially adds two periodic signals resulting in a single signal.Two switches 290, 291 couple the 180° signal splitter/combiner 222 andthe 0° splitter/combiner 224 to a circuit probe 202 via coupledtransmission lines 231. Switth 283 is coupled to a 180° signalsplitter/combiner 262 and a 0° splitter/combiner 264 having similarfunctionality to the 180° signal splitter/combiner 222 and the 0°splitter/combiner 224 described above. Two switches 295, 294 couple the180° signal splitter/combiner 262 and the 0° splitter/combiner 264 to acircuit probe 204 via directional couplers 232. The directional couplers232 permit the tapping of signals originating at circuit probe 204.

Similarly, switch 272 is coupled to a pair of switches 282, 284. Switch284 is coupled to a 180° signal splitter/combiner 242 and a 0°splitter/combiner 244 having similar functionality to the 180° signalsplitter/combiner 222 and the 0° splitter/combiner 224 described above.Two switches 296, 297 couple the 180° signal splitter/combiner 242 andthe 0° splitter/combiner 244 to a circuit probe 204 via coupledtransmission lines 233. Switch 282 is coupled to a 180° signalsplitter/combiner 252 and a 0° splitter/combiner 254 having similarfunctionality to the 180° signal splitter/combiner 222 and the 0°splitter/combiner 224 described above. Two switches 293, 292 couple the180° signal splitter/combiner 252 and the 0° splitter/combiner 254 to acircuit probe 202 via directional couplers 230. The directional couplers230 permit the tapping of signals originating at circuit probe 202.

The measurement system 200 can be used to produce and measure signalswhich are used to calculate S-parameters in order to characterize adifferential circuit when interfaced via the circuit probes 202, 204 tothe differential circuit. As such, the measurement system 200 cangenerate differential and common mode signals having known attributesfor input into a differential circuit of a device under test (DUT).Similarly, the measurement system 200 can measure correspondingdifferential and common mode signals present at one or more ports of theDUT. Consequently, the S-parameters may be calculated as will bedescribed below. For example, to produce a differential mode signal forinput via circuit probe 202, switch 280 is positioned to couple the 180°splitter/combiner 222 to the NWA, and switches 290 and 291 positioned tocouple the 180° splitter/combiner to the circuit probe 202. The NWAgenerates a signal, such as a radio frequency (RF) signal, which issplit by the 180° splitter/combiner 222 to produce a differential modesignal having signal components having substantially equal amplitudesand having a phase difference of substantially 180°. The differentialmode signal is transmitted through the transmission line and launchedinto a port of a DUT via the circuit probe 202. By selecting the 180°splitter/combiner 242 or the 0° splitter/combiner 244, differentialsignals and common mode signals, respectively, may be measured at port210. A reflected signal having a mode (differential or common)corresponding to the input signal is measured through the NWA port 209,210 from which the input signal is sent. For example, to generate adifferential mode signal for input from port 209 through circuit probe202, switches 270, 280, 290, and 291, are set such that a path throughthe 180° splitter/combiner 222 is chosen. This setting also allowsmeasurement of the reflected differential mode signal returning throughprobe 202. However, to measure the reflected common mode signal usingthe same input, the measurement is taken at NWA port 210 by settingswitches 272, 282, 292, and 293, such that a path through the 0°splitter/combiner 254 is enabled. One skilled in the art wouldappreciate that by varying the combination of switches and signalsources (NWA ports 209, 210), both transmitted and reflecteddifferential and common mode signals may be measured. Using thesemeasurements, all mixed mode S-parameters can be calculated as definedabove.

THE CIRCUIT PROBE

According to the present invention, a circuit probe is provided forinterfacing the measurement system to the differential circuit of a DUT.The probe incorporates a coupled pair of transmission lines to conveysignals having at least two modes of propagation between the measurementsystem and the differential circuit. Traditionally, transmission lineshave been formed such that the even-mode characteristic impedance ismuch larger than the odd-mode characteristic impedance, i.e., Z_(e)>>Z_(o). With this relationship, the odd-mode propagation is favoredover even-mode propagation, due to the mismatch loss of even-mode waves;therefore the differential mode signal is favored. The transmission linepair of the present invention is designed such that the even and oddmodes propagate with substantially equal characteristic impedances,i.e., Z_(e) =Z_(o). The use of a transmission line having Z_(e) =Z_(o)in the probe is counter-intuitive to prior art design guidelines.However, by using a transmission line with these characteristics, theimplementation and use of the measurement system is facilitated.

In the preferred embodiment, the circuit probe is designed forinterfacing with a two-port differential circuit implemented on a waferor other integrated circuit medium. Referring to FIG. 3, a fragmentarytop view of a circuit probe 300 is shown, in accordance with the presentinvention. The circuit probe 300 is adaptable to function as the circuitprobes 202, 204, used in the measurement system 200 described above. Inthe preferred embodiment, the probe has four contacts 312, 314, 316,318. These contacts comprise two ground contacts 312, 318 at opposingends of the probe, and two coupled signal contacts 314, 316 positionedbetween the two ground contacts. The two signals contacts forms coupledsignal paths. The coupled signals paths are balanced, such as beingsymmetrical, and the probe allows both even and odd modes to propagatein an approximately loss-less fashion. Furthermore, the even and oddmodes propagate with welt-defined characteristic impedances.

FIG. 4 is a second fragmentary view of the circuit probe 300 showing thebottom or interface surface. FIG. 5 is a fragmentary cross-sectionalview of the circuit probe 300. FIG. 6 is an enlarged fragmentary viewshowing dimensional characteristics of a transmission line of thecircuit probe 300. Referring to FIGS. 3-6, at least a portion of thecircuit probe 300 of the preferred embodiment is implemented on analumina substrate 305 having a relative dielectric constant between 9.6and 9.9. The alumina substrate is planar and has first and secondopposing surfaces 320, 322. A coupled mode transmission line 325 isimplemented on the alumina substrate such that signals propagating onthe transmission line 325 will have equal or substantially equal evenand odd-mode impedances. The transmission line is formed by disposingadjacent signal conductors 334, 336 in a coupled microstrip arrangementon a first plane 321 at or near the first surface 320 of the aluminasubstrate 305. The signal conductors 334, 336 of the coupled microstripare symmetrical, and each signal conductor has a thickness, T, and awidth, W. Each signal conductor 334, 336 is spaced apart from each otherby a spacing, S. The signal conductors 334, 336 are disposed between twocoplanar ground conductors 332, 338, the two coplanar ground conductors332, 338 forming a coplanar ground plane. Each ground conductor 332, 338has substantially the same thickness as the signal conductor, T.Additionally, each ground conductor 332, 338 has a width, W_(gnd) and isspaced apart from the closest signal conductor by a spacing, S_(gnd). Asecond ground plane 327 is disposed on a second plane 323, at or nearthe second surface 322 of the alumina substrate 305, and parallel to thecoupled microstrip 334, 336 and to the coplanar ground conductors 332,338. The second ground plane 327 is electrically coupled to the coplanarground conductors 332, 338, by conductive vias 328. The first and secondparallel planes 321, 323 are spaced apart by a spacing, H, and areseparated by alumina substrate material 305. According to the presentinvention, design parameters have been developed for the transmissionline 325, including the coupled microstrip 334, 336, the coplanar groundconductors 332, 338, and the bottom ground plane 327, relating to thespatial and dimensional characteristics of each.

The design parameters are defined with respect to a 50 ohm impedancesystem. The thickness, T, should be substantially less than H, i.e.,##EQU3## The ratio of W_(gnd) to S_(gnd) should be such that: ##EQU4##The following design parameters are based in part on the relativedielectric constant, ε_(r) of the substrate 305 used in constructing thetransmission line 325. For a given relative dielectric constant, ε_(r),W/H should be in the range defined below to ensure the return loss (intoZ_(e) =Z_(o) =50Ω) of the differential and common modes are both atleast 25 dB.

Thus, ##EQU5## where,

    (W/H).sub.MAX =1/(0.520ε.sub.r.sup.0.47 +0.011ε.sup.1.65)

    (W/H).sub.MIN =1/(0.300ε.sub.r.sup.0.40 +0.012ε.sup.1.80)

A graph reflecting (W/H)_(max) and (W/H)_(min) versus the substraterelative dielectric constant ε_(r) is shown in FIG. 7. For a givenrelative dielectric constant, ε_(r), the ratio of W/S should be lessthan or equal to the maximum value (defined below) to ensure the returnloss (into 50Ω) of the differential and common modes are both at least25 dB. Thus, ##EQU6## Where,

    (W/S).sub.MAX =11.0 /(ε.sub.r .sup.1.30).

A graph reflecting (W/S)max versus the substrate relative dielectricconstant ε_(r) is shown in FIG. 8. For a given relative dielectricconstant, ε_(r), the ratio of W/S_(gnd) should be less than or equal toa maximum value to ensure the return loss (into 50Ω) of the differentialand common modes are both at least 25 dB. Thus, ##EQU7## where,

    (W/S.sub.gnd).sub.MAX =80/(ε.sub.r.sup.1.5).

A graph reflecting (W/S_(gnd))_(max) versus the substrate relativedielectric constant ε_(r) is shown in FIG. 9.

The design parameters described above can be used in conjunction toprovide for an even mode and odd mode propagation having low insertionloss (less than 0.1 dB), and a high return loss (greater than 30 dB)into a 50 ohm load. The above parameters are scalable, thus enablingvarious embodiments of the physical probe. In the preferred embodiment,the substrate 305 has ε_(r) of approximately 9.6, and the designparameters are such that, T<H/10; W/S≈0.2; W/H≈1.0; W/S_(gnd) ≈1.0; andW_(gnd) /S_(gnd) >=1.

METHOD OF OPERATION

Ordinarily, in a two-port system, the signals are launched at one portand measured at both ports. The measurements encompass the differentialand common signals detected at each port, including the input port, whena differential mode signal or a common mode signal is launched at theinput port. The combiners are selectively switched in at each port suchthat the signals present on the coupled pair of transmission lines arecombined and presented at NWA ports for analysis.

The measurement system can be operated to determine S-parameters, asdescribed above, for the characterization of a differential circuit fora DUT. Referring to FIG. 10, the procedures for determining theS-parameters are summarized. An input differential wave is introduced orinjected into a port of the differential circuit, step 1010. The inputdifferential wave is provided via a signal generated by the NWA, whichis routed through the 180° splitter/combiner, and propagated over acoupled pair of transmission lines to a port on the differentialcircuit. While introducing the input differential wave, a firstdifferential output wave is measured, step 1020. Also while introducingthe input differential wave, a first common mode output wave ismeasured, step 1030. Likewise, an input common mode wave is introducedinto the differential circuit, step 1040. The input common mode wave isprovided via a signal generated by the NWA, which is routed through the0° splitter/combiner, and propagated over a coupled pair of transmissionlines to a port on the differential circuit. While introducing the inputcommon mode wave, a second differential output wave is measured, step1040. Also while introducing the input common mode wave, a second commonmode output wave is measured, step 1050. This process is repeated ateach port in an N-port device. Preferably, output waves are measured atall ports, including the input port. In a two-port device, there will besixteen (16) combinations of input and output signals for calculatingthe S-parameters to fully characterize the differential circuit.

CONCLUSIONS

According to the present invention, full characterization of a deviceunder test (DUT) can be accomplished automatically with only two probes.Full characterization includes differential S-parameters, common modeSparameters, and cross-mode (or mode-conversion) S-parameters. Fullknowledge of all parameters, especially the cross-mode, is a significantaspect of the present invention as it relates to error correction andcalibration. Consequently, the accuracy of the measurement system isenhanced.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as defined by theappended claims.

What is claimed is:
 1. A circuit probe particularly applicable formeasuring a differential circuit, the circuit probe comprising:atransmission line comprising:first and second ground conductors; andfirst and second coupled signal conductors adjacently disposed betweenthe first and second ground conductors; the signal conductors and groundconductors being formed to propagate signals with substantially equaleven-mode and odd-mode characteristic impedances; first and secondground contacts coupled to the first and second ground conductors,respectively; and first and second signal contacts coupled to the firstand second signal conductors, respectively.
 2. A circuit probeparticularly applicable for measuring a differential circuit, thecircuit probe comprising:a transmission line comprising:first and secondaround conductors; and first and second coupled signal conductors,wherein the first and second coupled signal conductors are disposedbetween, and on a first plane substantially coplanar with, the first andsecond ground conductors; the signal conductors and ground conductorsbeing formed to propagate signals with substantially equal even-mode andodd-mode characteristic impedances; first and second ground contactscoupled to the first and second ground conductors, respectively; firstand second signal contacts coupled to the first and second signalconductors, respectively; a ground plane disposed substantially parallelto the first plane, the ground plane being electrically coupled to theground conductors; and a substrate disposed between the signalconductors and ground conductors, and the ground plane.
 3. The circuitprobe of claim 2, wherein:the substrate has a relative dielectricconstant ε_(r) ; each ground conductor has a width, W_(gnd) ; eachsignal conductor has a width W, and is separated from each other by aspacing, S, and is separated from one of the ground conductors by aspacing S_(gnd) ; the ground conductors and signal conductors areseparated from the ground plane by as spacing, H; wherein: ##EQU8##where,

    (W/H).sub.MAX =1/(0.520ε.sub.r.sup.0.47 +0.011ε.sub.r.sup.1.65)

    (W/H).sub.MIN =1/(0.300ε.sub.r.sup.0.40 +0.012ε.sub.r.sup.1.80).


4. The circuit probe of claim 3, wherein: ##EQU9## where,

    (W/S).sub.MAX =11.0/(0.740ε.sub.r +0.20ε.sub.r.sup.2 -0.011ε.sub.r.sup.3).


5. 5. The circuit probe of claim 4, wherein: ##EQU10## where,

    (W/S .sub.gnd).sub.MAX =80 /(ε.sub.r.sup.1.5).


6. The circuit probe of claim 5, wherein: ##EQU11##
 7. The circuit probeof claim 2, wherein:the substrate comprises alumina; each groundconductor has a width, W_(gnd) ; each signal conductor has a width W,and is separated from each other by a spacing, S, and is separated fromone of the ground conductors by a spacing S_(gnd) ; the groundconductors and signal conductors are separated from the ground plane byas spacing, H; and wherein:W/S is substantially equal to 0.2; W/H issubstantially equal to 1.0; W/S_(gnd) is substantially equal to 1.0; andW_(gnd) /S_(gnd) >=1.
 8. A transmission line comprising:first and secondcoupled signal conductors; first and second coplaner ground conductors,the first and second coupled signal conductors being adjacently disposedbetween, and being substantially coplanar with the first and secondground conductors; a ground plane disposed substanially parallel to theground conductors and signal conductors, the ground plane beingelectrically coupled to the ground conductors; and a substrate disposedbetween the signal conductors and ground conductors, and the groundplane; wherein the signal conductors and ground conductores propagatesignals with substantially equal even-mode and odd-mode characteristicimpedances.
 9. The transmission line of claim 8, wherein:the substratehas a relative dielectric constant ε_(r) ; each ground conductor has awidth, Wg_(nd) ; each signal conductor has a width W, and is separatedfrom each other by a spacing, S, and is separated from one of the groundconductors by a spacing S_(gnd) ; the ground conductors and signalconductors are separated from the ground plane by as spacing, H;wherein: ##EQU12##

    (W/H).sub.MAX =1/(0.520ε.sub.r.sup.0.47 +0.011ε.sub.r.sup.1.65)

    (W/H).sub.MIN =1/ (0.300ε.sub.r.sup.0.40 +0.012ε.sub.r.sup.1.80).


10. The transmission line of claim 9, wherein: ##EQU13## where,

    (W/S).sub.MAX =11.0/(ε.sub.r.sup.1.30).


11. 11. The transmission line of claim 10, wherein: ##EQU14## where,

    (W/S.sub.gnd).sub.MAX =80/(ε.sub.r.sup.1.5).


12. The transmission line of claim 11, wherein: ##EQU15##
 13. Thetransmission line of claim 8, wherein:the substrate comprises alumina;each ground conductor has a width, W_(gnd) ; each signal conductor has awidth W, and is separated from each other by a spacing, S, and isseparated from one of the ground conductors by a spacing S_(gnd) ; theground conductors and signal conductors are separated from the groundplane by as spacing, H; andwherein: W/S is substantially equal to 0.2;W/H is substantially equal to 1.0; W/S_(gnd) is substantially equal to1.0; and W_(gnd) /S_(gnd) >=1.